The via2 structure in digital micromirror devices (DMD), which provides a connectivity path for connecting the underlying memory cells to their respective address electrodes on the metal-3 layer, can indirectly cause a dimple in the metal surface of the rotating beam. These dimples reduce the clearance between the beam and address electrode when the beam is rotated (tilted). This can result in the beam shorting to the address electrode.
FIG. 1 shows the build-up of a DMD starting with the memory layer 10, the metal-3 layer 11, the beam layer 12, and the mirror layer 13. The memory 10 is an array of binary cells 100 representing pixels. The metal-3 layer 11 is comprised of positive and negative beam address electrodes 110,111 and a beam landing pad plane 112. A thick oxide layer 101 isolates the metal-3 layer from the memory layer and also has via2 paths 102,103 connecting the memory's  and  output signals to the positive and negative address electrodes 110,111 at contacts 113,115, respectively.
The beam layer 12 is comprised of a rotating beam 121 (also called yoke) attached by means of thin torsion hinges 120 to beam post caps 126, which are supported above the landing pad plane 112 on the metal-3 layer 11 by beam posts 127. Also on this layer are mirror address electrodes 123,124 supported above the beam address electrodes 110,111 by support posts 125,128 so that the beam and mirror address electrode pairs 110/123 and 111/124 are at the same  and  potential, respectively. Finally, the highly reflective mirror 130 sits on top of the rotating beam 121, supported by the mirror post 131.
In operation, electrostatic forces cause the mirror/beam structure to rotate on its torsion axis, defined along the torsion hinges 120. These electrostatic forces are established by the voltage potential difference between the beam address electrodes 110/111 and the beam 121 and between the mirror address electrode 123/124 and the mirror 130, respectively. In each case, these forces are a function of the reciprocal of the distance between the two plates; e.g., 110/121 and 123/130. As the rigid beam/mirror structure rotates on its axis, the torsion hinges 120 resist deformation with a restoring torque that is an approximate linear function of the angular deflection of the structure. The structure rotates until either this restoring torsion beam torque equals the established electrostatic torque or until the beam/mirror structure is mechanically limited in its rotation, i.e., the beam tips 122 contact the landing pads 112. In typical DMD display applications, the structure is such that it is mechanically limited in its rotation (lands), so as to provide binary states at approximately+/−10° each.
Planarization, or smoothness of surfaces within the device is critical, both in the mirror for optical reasons and in the beam and other surfaces for reliability reasons. In fabricating these devices a sacrificial layer of appropriate thickness is deposited over the metal-3 layer and then the beam metal layer is fabricated over this sacrificial photoresist layer. The sacrificial layer is then removed freeing the beam structure 121 to rotate on it's torsion hinges 120 through a positive or negative angle until the beam tips 122 land on the landing pads 112. One problem in these type of devices is that when the beam metal is put down it will tend to follow the contour of the via2 transferred through the sacrificial photoresist layer, leaving an indent or dimple 140,141 in the beam over the via2 area once the sacrificial photoresist is removed, thus leaving a degradation in the beam planarity in these regions. These dimples can become sufficiently large that when the beam tilts, the dimple will contact the underlying beam address electrode, which is at a different electrical potential, before the beam tips land, thereby shorting out the device. This failure mechanism can significantly lower the yield of the devices.
What is needed is a device with very high beam planarity such that the beam tips are assured of landing prior to any other contact between device elements. The device of the present invention addresses the relationship between the dimples in the beam created over the via2s and assures that the clearance is such that these are much less likely to short out prior to the beam landing.